The relative inefficiency of photovoltaic cells has been one significant factor limiting the range of applications for which photovoltaic cells are a commercially viable source of alternative energy. One cause of inefficiency is reflection, across the pertinent part of the solar spectrum, from the front surface of the semiconductor film that contains the photovoltaic junction region. Monocrystalline silicon, for example, generally exhibits a reflectivity above 30% for quasi-normal incidence.
Several approaches have been tried for reducing the surface reflectivity of silicon films. One approach involves surface modification through reactive-ion etching, leading to a product referred as “black silicon”, having a reflectivity that for quasi-normal incidence is reduced to values near 5%. Another approach is to overcoat the silicon surface with interference-type layers of dielectrics such as silicon nitride. The resulting antireflection coatings are effective for selected wavelengths.
Recently, P. Spinelli et al., “Broadband omnidirectional antireflection coating based on subwavelength surface Mie resonators,” Nature Communications 3, Article No. 692 (Feb. 21, 2012), hereinafter “Spinelli et al.”, reported a new type of antireflection coating using Mie resonators. A Mie resonance in the electromagnetic scattering cross section of a particle (Mie scattering theory is typically applied to spherical and cylindrical particles) is a value indicative of particularly strong or particularly weak scattering at a wavelength comparable to the geometrical size of the scattering particle.
Spinelli et al. found computationally and experimentally that a two-dimensional lattice of silicon nanocylinders of sub-wavelength dimensions fabricated on a silicon surface would exhibit Mie resonances strongly coupled to the underlying silicon substrate. Forward scattering from the nanocylinders, acting as Mie resonators, strongly suppressed reflection from the silicon surface. Pronounced line broadening was attributed to the electromagnetic coupling to the substrate, which provided a dissipative channel for radiation confined within the nanocylinders. Because of the line-broadening effect, the antireflective effect was observed to extend across a wide spectral range.
Another study of the electromagnetic scattering properties of silicon nanocylinders, useful in the present context, is reported in I. Staude et al., “Tailoring Directional Scattering through Magnetic and Electric Resonances in Subwavelength Silicon Nanodisks,” ACS Nano 7 (9), (published online Aug. 16, 2013), 7824-7832 (hereinafter, “Staude et al.”).
The entirety of Spinelli et al. and the entirety of Staude et al. are hereby incorporated herein by reference.
Although the anti-reflective approaches described above are useful for improving the efficiency of photovoltaic cells, there remains a need for new approaches that can drive up the efficiencies still further.